Method for decoding and coding a compressed video data stream with a reduced memory requirement
The present standards for compressing video data, MPEG1 and MPEG2, operate in accordance with the principles of prediction and transformation. A distinction is made between three differently coded types of pictures. So-called I pictures (intra-coded pictures) are only transform-coded. The transform used is the discrete cosine transform (DCT). P pictures are predicted from the respectively preceding I picture or P picture. The difference between the predicted picture and the actual picture is DCT-transformed. The third type of pictures are the so-called B pictures, which are bidirectionally predicted from the respectively preceding I picture or P picture and from the respectively succeeding I picture or P picture. The difference between the predicted picture and the actual picture is again DCT-transformed.
An MPEG decoder must therefore make available the data of two pictures (I picture and P picture) for the reconstruction of the B pictures. Accordingly, the memory requirement for pictures is high.
The fundamental mode of operation of the compression methods MPEG1 and MPEG2 has been disclosed to the person skilled in the art by various publications (D. J. Le Gall, The MPEG Video Compression Algorithm, Signal Processing: Image Communication 4, pp. 129-140, 1992; International Standard ISO/IEC 11172-2: Coding of Moving Pictures and Associated Audio, ISO/MPEG, 1993 and Draft International Standard ISO/IEC 13818-2, Generic Coding of Moving Pictures and Associated Audio, 25.3.1994, 1994).
In order to improve the quality of the prediction in those picture areas in which moving objects occur, use is made of so-called motion-compensated prediction. The principles of the motion estimation required for this purpose and their application for the motion-compensated prediction have been disclosed to the person skilled in the art, for example by (M. Bierling, Displacement estimation by hierarchical block matching, 3rd SPIE Symp. on Visual Communications, Cambridge, Mass., November 1988, 1988) and (Draft International Standard ISO/ICE 13818-2, Generic Coding of Moving Pictures and Associated Audio, 25.3.1994, 1994).
A distinction is made between three differently coded types of pictures. So-called I pictures are transmitted without any chronological prediction, but rather are subjected only to intra-picture coding, preferably DCT coding with subsequent quantization of the coding transform coefficients. In the context of this patent application, "intra-picture coding" is to be understood quite generally as any method which is suitable for treating local correlations in video data. The so-called P pictures are predicted with the aid of the DPCM loop from chronologically preceding I pictures or P pictures (forward prediction). The difference between the predicted picture and the actual picture is subjected to intra-picture coding, preferably to transformation using a DCT with subsequent quantization of the coding transform coefficients. So-called B pictures, which are also designated as interpolated pictures in the context of the present patent application, are chronologically situated between an I picture and a P picture or between two P pictures. B pictures are determined by means of (bidirectional) motion-compensated interpolation from a chronologically preceding I picture or P picture and from a chronologically succeeding I picture or P picture. In this case, the expressions (chronologically) "succeeding" and "preceding" do not refer to the order in which these pictures are transmitted in the video data stream of the compressed pictures, but rather they refer to the order in which these pictures are recorded/reproduced. In the same way as P pictures, B pictures, too, are coded in the form of quantized coding transform coefficients of a difference picture.
In the case of currently known implementations, the reconstruction of a B picture by means of motion-compensated interpolation from a chronologically preceding I picture or P picture and from a chronologically succeeding I picture or P picture necessitates the provision of the two reference pictures (which are also occasionally designated as support pictures in the literature) in fully decoded form.
Therefore, two fully decoded reference pictures (I pictures or P pictures) have to be stored in a frame store in the case of the methods belonging to the prior art for carrying out motion-compensated interpolation.
The re-interlacing during the video output requires further storage capacity. The overall required memory is a decisive cost factor in the hardware used for decoding and encoding. A reduction in the storage capacity required is therefore desirable.